Ayrton Senna died on 1 May 1994. What followed was a grim afterword to the great Brazilian's life story: the search for an explanation and, ultimately, for someone to blame for his death.

On 1 May 1994, Ayrton Senna veered off the track at the Imola circuit and into the crash barriers. No one knew it at the time, but the great Brazilian was effectively killed on impact: debris had pierced his helmet, causing multiple fractures at the base of his skull. The medics who arrived on the scene instantly knew that his chances of survival were zero and that Senna was gone, perhaps to meet the God he believed in so fervently. What now began was a grim afterword to the great Brazilian's life story: the search for an explanation and, ultimately, for someone to blame.

Officially, the three-time world champion died at 18:37 local time. Later that evening, Williams team manager Ian Harrison received a phone call from an Italian lawyer. He was informed that Senna's death was being treated as a "road traffic accident".

Publicly, the immediate aftermath focussed on the sense of loss that Senna's death caused, both in the sport and in his native Brazil. The man acknowledged as the best of his generation and among the greatest of all time was gone, brutally and without warning, becoming the first (and to date only) F1 champion to die in a world championship grand prix. What's more, he perished in an age of live television coverage across the globe; F1 had never experienced a more public death than Senna's. As if to confirm this, his state funeral was broadcast live on television in Brazil, while the government declared three days of national mourning.

But as the grief subsided, it became apparent that Senna's death was being treated as particularly suspicious. All fatal racing crashes require intense scrutiny. Even if it is clear that the accident occurred due to driver error, the authorities must investigate extensively so as to learn lessons that can be implemented in the future. This was the case with Roland Ratzenberger, whose death the day before Senna's had begun F1's darkest weekend. Ratzenberger had gone off track during qualifying, damaging his front-wing, but continued pushing for a quick lap. The damaged wing failed and the Austrian crashed heavily, suffering fatal head injuries. Put very simply, driver error had set in motion a chain of events that led to Ratzenbeger's crash. Blame, for want of a better word, could be apportioned.

Senna's car is removed from the scene of the accident // PA Images

But from the beginning it was felt that Senna's accident was unusual. On-board footage showed his Williams-Renault seeming to snap to the right while he negotiated the left-hand Tamburello corner. He was not able to significantly adjust the angle of impact and hit the wall almost head on. In this respect, and based only on a superficial viewing, it looks more like a mechanical failure than driver error. Senna was by no means beyond mistakes, but this was a strange accident for a man of his vast experience, and one in which the car appeared to behave unusually.

It would eventually emerge that Senna's steering column had been identified as a potential cause for the crash. There was reason behind this, too. The Brazilian had wanted the position of his steering wheel changed, so it had been extended by welding and reinforcing additional length to the existing column. This work was carried out with the approval of Williams director of engineering Patrick Head, and the team's chief designer Adrian Newey.

It was this that led to team boss Frank Williams, as well as Head and Newey, all being charged with manslaughter. So too were three race organisers. If convicted, Williams and his colleagues faced anything from a two-year suspended sentence to a seven-year jail term. Senna's accident happened in seconds, but the search for blame would take many years.

Imola is a town of just under 70,000 people in the Emilia-Romagna region of Italy. For many years it staged the San Marino Grand Prix, though the tiny landlocked principality is actually an hour up the road. The circuit's official name is Autodromo Enzo e Dino Ferrari, though most people simply use Imola for both town and track. Beside hosting the race for a few days each year, it was not accustomed to global attention. Beginning in February 1997, however, its district courthouse would attempt to apportion blame for the death of the world's most famous racing driver.

Though there are some wounds that will never fully heal, F1 had to some degree moved on from Senna's death by this point. Michael Schumacher had become the sport's new star, the Williams team had regrouped and won world titles, and a raft of improvements were making the sport fundamentally safer. Newey was not even a Williams employee anymore, having switched to McLaren for the 1997 campaign.

And so the drawn-out trial process always felt like an anachronism, even if it was necessary under Italian law. What's more, team chiefs being placed on trial was alien to Formula 1. Many drivers had died prior to Senna, but the aftermath had been different: the team investigated the cause of the accident with outside assistance, before lessons were learnt internally and then shared throughout the sport as required. But times had changed and this was no longer acceptable protocol. Senna was F1's first race weekend fatality in 12 years, and this was reflected in the very public nature of his trial.

The chief allegation being brought against the team was that the steering column had broken at the point where modifications had been made, leading directly to Senna's fatal crash. The team accepted that this failure had occurred – after all, there was a broken steering column to prove it – but argued that it had done so on impact with the wall. In other words, the crash caused the column to break, not the other way around.

An extensive list of witnesses testified, including reigning world champion Damon Hill, who had been Senna's teammate that day at Imola. The Englishman did not subscribe to the steering column theory, reasoning that something else must have happened to cause Senna to leave the track. Subsequently, Hill has stated that simple driver error was the most likely explanation.

Newey (left) and Head at the launch of the 1994 Williams-Renault // PA Images

Though anathema to some of his fans, this was raised at the trial as a potential explanation for the crash. The Williams lawyers also suggested Senna may have been attempting to avoid a piece of debris on the track, following an accident at the start of the race, and lost control as a result. It is also a fact that his tyres would have been cold after several laps behind the safety car, which was deployed following the original crash. Ultimately, there were a number of possible explanations that did not involve a steering column failure; while it was a possibility, plausible alternatives existed.

But a technical report submitted by Bologna University shifted the emphasis back to the steering column. Led by Professor Enrico Lorenzini, it stated that the column had been "badly welded together about a third of the way down and couldn't stand the strain of the race... I believe the rod was faulty and probably cracked even during the warm-up. Moments before the crash only a tiny piece was left connected and therefore the car didn't respond in the bend."

The emphasis now shifted almost entirely to Head and Newey, with state prosecutor Maurizio Passarini recommending that Frank Williams and the three race officials be cleared of the charges.

But the judge was not satisfied that the prosecution had proven their case sufficiently, with the debris-on-track claim given added validity by a new photograph that surfaced just as the trial was getting underway. The judge concluded that while the column had likely failed, there was no conclusive proof that negligence on the part of the team had caused this. On 16 December 1997, Williams, Head and Newey were acquitted of the charges. The race organiser, circuit manager and race director Roland Bruynseraede were also exonerated.

"I would think they'll be very pleased that this part of the process is over and that it has gone successfully for them," the team's lawyer said following the verdict. But for two of those involved there was more to come.

Frank Williams and the three race organisers were now in the clear, but prosecutors continued to push for one-year suspended sentences for Head and Newey. A retrial was ordered, and so the case rumbled on for two more years, during which the Newey's McLaren cars became the dominant force in F1 while his old team's fortunes waned. On 22 November 1999, an appeals court upheld the original verdict; once again, no blame could be apportioned.

The trial continued to drag on, however, with the possibility of a return to the courtroom seeming to crop up every few months in the motorsport press. The pace of progress was glacial; it was not until April 2002 that Italian police finally returned Senna's car to the Williams team, by which time it was in an advanced state of deterioration. Unable to learn much from the old chassis, they had it destroyed soon after.

Hopes that this was the end of the story were scuppered in January 2003 when the Italian Supreme Court reopened the case once more, citing "material errors" in the original process. In May 2005, more than 11 years after Senna's death, Newey was finally fully acquitted.

Head, however, was ruled to be responsible for the faulty steering column, but the case had timed out under Italy's statute of limitations. This is set at seven years and six months, while the eventual verdict came in 2007. It concluded: "... the accident was caused by a steering-column failure. This failure was caused by badly designed and badly executed modifications. The responsibility of this falls on Patrick Head, culpable of omitted control."

Yet the crucial fact was never proved: did Senna's steering column fail before or after the impact? With the car gone, the exact reason for his death may never be entirely clear.

A statue of Senna in a park near the Imola circuit, adorned with his famous blue cap and a Brazilian flag // PA Images

In 2013 Newey told BBC Sport that the accident "still haunts me to this day." He added that "no one will know" exactly what caused Senna to crash, acknowledging the problems with the steering column and uncertainty over just when it broke. It has always been clear that Newey – a sensitive man, as well as the greatest technical mind of his generation – was deeply affected by Senna's death. He wrestled with whether he should remain in the sport, ultimately choosing to do so.

It should be added that the general attitude within the sport was, unsurprisingly, against the trial ever taking place. Senna's own family were against the legal process; Niki Lauda called it a "cynical and stupid exercise"; and Max Mosley, the FIA president from 1993 to 2009, felt it slowed progress. "Nothing useful came out of the legal case," Mosley is quoted as saying. "In fact, it was a great hindrance to any work on safety, because by impounding the car and all the equipment and everything relevant thereto, they prevented us from looking at it, and just wasted everybody's time."

Years later, Hill explained to The Times: "I have listened [to] and read endless theories about why, or how, he could have crashed on such a 'simple' corner like Tamburello. No one other than Ayrton Senna and me know what it was like to drive that car, through that corner, in that race, on that day, on cold tyres.

"Ultimately we will never know what Ayrton was thinking, or what really happened," Hill continued. "I am convinced that he made a mistake, but many people will never believe he could. Why not? He made many mistakes in his career."

Controversy was rarely far away during Senna's career, and the same was true for many years after his death. While we must accept Italy's legal requirements, it can be said that the trial turned Senna's death into a drawn out process. What should have faded into the past, to be replaced by the memories of what he achieved on track, remained grimly present. Senna may have died on 1 May 1994, but his death felt like a current event for more than a decade afterwards. It was not until the trial process was fully exhausted that the sport was able to draw a line under Senna's death, and focus instead on what he achieved during his life.